TOKYO, June 8 -- A North Korean court sentenced two U.S. journalists Monday to 12 years in a labor camp, as the government of Kim Jong Il continued to ratchet up tension with the United States and its neighbors.

Laura Ling and Euna Lee received harsher sentences than many outsiders expected. But several experts in South Korea predicted that talks would begin soon to negotiate their release.

Ling, 32, and Lee, 36, were working for Current TV, a cable and Web network co-founded by former vice president Al Gore, when they were detained March 17 by North Korean soldiers along the border with China. The reporters were working on a story about North Koreans who flee the country, but the circumstances of their arrest are not clear.

"Obviously, we are deeply concerned about the length of the sentences and the fact that this trial was conducted totally in secret, with no observers, and we're engaged in all possible ways through every possible channel to secure their release," Secretary of State Hillary Rodham Clinton said in Washington. "And we once again urge North Korea to grant their immediate release on humanitarian grounds."

The five-day trial was held in Pyongyang's Central Court, the top court in North Korea. "The trial confirmed the grave crime they committed against the Korean nation and their illegal border crossing," the official Korean Central News Agency said.

The detention and sentencing of the two journalists has coincided with, and become entangled in, a series of acts by North Korea this spring that have alarmed its neighbors and even its historical allies. The heavily armed, secretive state -- in the throes of a succession process, as the country's ailing leader prepares to hand power to his youngest son -- launched a long-range missile in April, detonated a nuclear bomb in May and has renounced the truce that ended the Korean War.

On Monday, North Korea warned fishermen and boat captains to stay away from the country's east coast, Japan's coast guard said. The North is planning to launch several medium-range missiles from the region, according to reports in the South Korean news media.

The U.S. government, which last year lifted some sanctions against North Korea and delivered large amounts of food aid, has become increasingly exasperated by the North's behavior. President Obama, who came into office saying he was prepared to meet personally with Kim Jong Il, said Saturday that "we are going to take a very hard look at how we move forward on these issues."

Led by the United States and Japan, the U.N. Security Council is considering new sanctions against North Korea for exploding a nuclear device in defiance of U.N. resolutions.

North Korea threatened Monday that if the sanctions are approved, it will retaliate with "extreme" measures. "Our response would be to consider sanctions against us as a declaration of war and answer it with extreme hard line measures," the North's official Rodong Sinmun newspaper said in a commentary.

The long prison term given the journalists adds a new complication.

The verdict cannot be appealed and is final, officials in Seoul have said. But there is a widespread expectation, at least in South Korea, that the journalists will be released when the North Korean government decides the time is right to talk again to the United States.

In the past, North Korea has released Americans who have entered the country illegally. The government also has a history of brinkmanship, turning confrontation and bluster into negotiations that reward it with food, fuel and other concessions.

"The verdict does not mean much, since they will get released," said Andrei Lankov, an expert on North Korea who teaches at Kookmin University in Seoul. "Unfortunately, right now the North Koreans want to keep tensions high, so it will take many months and perhaps a year or more before the Pyongyang authorities will decide that it's time to make some friendly gesture to Washington."

Other North Korean observers were more hopeful of a quick resolution.

"Now that they are sentenced, we can think and talk about making arrangements for their release," said Han Seung-soo, a former South Korean foreign minister. "It is ironic, but with the sentencing we now have something more tangible to negotiate about."

The families of the women were "shocked and devastated" by the sentence, according to a statement issued through their spokeswoman. They asked the North Korean government "to show compassion" and grant the women clemency.

"We believe that the three months they have already spent under arrest with little communication with their families is long enough," the families said.

Laura Ling suffers from an ulcer, her sister said. Lee has a 4-year-old daughter.

Both women have been allowed to telephone their families in the United States, which is highly unusual in a state that seals away political prisoners in concentration camps, sometimes for life, without any contact with relatives.

Ling and Lee have been visited three times by the Swedish ambassador and have been allowed to send and receive letters through him. The United States does not have diplomatic relations with North Korea.

Staff writer Glenn Kessler in Washington and special correspondent Stella Kim in Seoul contributed to this report.